1. Field of the Invention
The present invention relates to a post-processing apparatus and a post-processing method to form a fold line for saddle stitching in a sheet on which an image is formed.
2. Description of Related Art
When an image forming apparatus is connected to various types of post-processing apparatuses, various bookbinding processings can be realized, and thereby such an apparatus can be used as a printing apparatus.
For example, as shown in FIG. 18, a sheet is center-folded at the center a1 thereof and then this sheet is collected on a plural sheet basis to obtain a sheet bundle. Then, the fold line portion of the center a1 of this sheet bundle is stapled (ST of FIG. 18) and thereby saddle stitching bookbinding can be carried out.
Further, as shown in FIG. 19, the spine portion of a saddle-stitched sheet bundle is pressed with a roller and fold lines with corners b1 and b2 are made to produce a spine cover portion. Such operations make it possible to carry out square back processing allowing a sheet bundle to be in a state similar to perfect binding.
Incidentally, such saddle stitching and square back processing are proposed in Unexamined Japanese Laid-Open Patent Application No. 2009-120398.
In Japanese Laid-Open Patent Application No. 2009-120398 described above, the spine folded portion of the center of a sheet is nipped by rollers from both sides to carry out center folding for saddle stitching. In this case, the nipping pressures of the rollers are changed depending on execution or no execution of square back processing to carry out square bad processing favorably.
However, since such square back processing is carried out after saddle stitching, as shown in FIG. 20, a center folding trace a3 during saddle stitching remains in a portion of the spine cover. Therefore, in the square back processing, the quality as a booklet has been found to degrade.
Further, in the case where an image is formed on the spine cover portion, it has been found that a center folding trace (a3 in FIG. 20) deteriorates the image of the spine cover portion.
However, this square back processing presupposes saddle stitching. Therefore, if no center folding has been carried out, in conveyance from a folding section to a slacking section and a saddle stitching section and in positioning in the stacking section and the saddle stitching section, a new problem, in which stitching position accuracy is difficult to maintain, is posed.